Regenerative comparators, also known as Schmitt trigger circuits, have existed since 1938, when first implemented using tubes. The primary feature of a regenerative comparator is its ability to generate a clean rectangular pulse when supplied with a sine, saw tooth, or poorly-shaped rectangular pulse. The regenerative comparator is therefor, often used to clean up digital waveforms that have been corrupted by noise.
Another advantage of regenerative comparators is hysteresis. This allows the output to transition from a LOW level to a HIGH level when the input voltage reaches a first value and then transition back to the LOW level from the HIGH level when the input voltage reaches a second value.
Convention regenerative comparators are constructed from analog circuitry using a positive analog feedback loop. An example of a conventional regenerative comparator is shown in FIG. 1. Input voltage 22 is supplied to the positive input 30 of comparator 20. Negative input 32 of comparator 20 can be connected to either VREF1 on signal line 24 or VREF2 on signal line 26. Analog switch 28 determines whether negative input 32 is connected to signal line 24 or signal line 26. VREF1 and VREF2 are voltage levels where, in this example, VREF1 is greater than VREF2. Traditionally, VREF1 and VREF2 are related to each other in that they are both a function of a common voltage, such as a supply voltage. Output 34 of comparator 20 is fed back to analog switch 28 over line 36. When output 34 is LOW, for example 0 volts, analog switch 28 connects negative input 32 to signal line 24 and thus VREF1. When output voltage 34 is HIGH, for example 5 volts, analog switch 28 connects negative input 32 to signal line 26 and thus VREF2.
Thus, the circuit of FIG. 1 requires feedback to accomplish hysteresis. It is therefore not easy to fine tune the levels at which state changes are made, due to the feedback nature of the device. Also, since they are related, the hysteresis levels cannot be independently adjusted.